According to IMDb, there was no movie called 'Upstairs Downstairs' released in 1961. The closest I can find is "Upstairs and Downstairs", a comedy released in 1959, which towards the end of film features a train apparently leaving Kings Cross (although the station used for filming was actually Edinburgh Waverley). This is from the British Railway Movie Database:
https://ibb.co/JB1Vd0J